[Sema] Handle transparent_union attributes in C mode only

This commit marks the transparent_union attributes as C only because clang
doesn't support them in C++ mode. Prior to this commit, clang still tried to
verify these attributes in C++, leading to crashes when analyzing templated
transparent_union unions that have dependent field types. This commit ensures
that such crashes won't happen again.

As a result of this commit clang now displays a warning every time it encounters
a transparent_union attribute in C++ mode.

Differential Revision: https://reviews.llvm.org/D25308


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@283995 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Basic/Attr.td

@@ -1543,6 +1543,7 @@ def TransparentUnion : InheritableAttr {
   let Spellings = [GCC<"transparent_union">];
 //  let Subjects = SubjectList<[Record, TypedefName]>;
   let Documentation = [Undocumented];
+  let LangOpts = [COnly];
 }
 
 def Unavailable : InheritableAttr {

test/SemaCXX/attr-gnu.cpp

@@ -27,3 +27,19 @@ class D : public B {
   void test3() __attribute__((cf_unknown_transfer)) override {} // Ok, not known to GCC.
 };
 }
+
+template<typename T>
+union Tu { T b; } __attribute__((transparent_union)); // expected-warning {{'transparent_union' attribute ignored}}
+
+template<typename T>
+union Tu2 { int x; T b; } __attribute__((transparent_union)); // expected-warning {{'transparent_union' attribute ignored}}
+
+union Tu3 { int x; } __attribute((transparent_union)); // expected-warning {{'transparent_union' attribute ignored}}
+
+void tuTest1(Tu<int> u); // expected-note {{candidate function not viable: no known conversion from 'int' to 'Tu<int>' for 1st argument}}
+void tuTest2(Tu3 u); // expected-note {{candidate function not viable: no known conversion from 'int' to 'Tu3' for 1st argument}}
+void tu() {
+  int x = 2;
+  tuTest1(x); // expected-error {{no matching function for call to 'tuTest1'}}
+  tuTest2(x); // expected-error {{no matching function for call to 'tuTest2'}}
+}